Optical second harmonic generation ("SHG") using a non-linear medium such as a crystal or other similar means, provides a method for doubling, or more generally producing the N.sup.th harmonic of, the frequency of electromagnetic radiation emitted by a laser or other high intensity source. Other interactions in a nonlinear medium can produce new wavelengths of light by sum and difference freq. generation, parametric amplification, and related phenomena such as stimulated raman scattering. Second harmonic generation, using a ruby laser to produce light of wavelength substantially 3470 .ANG., was first reported by Franken, Hill, Peters and Weinreich in Phys. Rev. Letters 7 118 (1961). The theory of intra-cavity SHG has been discussed by R. G. Smith, I.E.E.E. Jour. of Quantum Electr. QE-6, 215-223 (1970) and by many others. Franken and Ward, in Rev. Mod. Phys. 35 23 (1963) has noted that a crystalline medium can produce a second harmonic of the frequency of the incident radiation only if the crystal lacks inversion symmetry so that the second order polarization tensor d.sup.ij in the lowest order non-linear polarization term, P.sub.(2) =.SIGMA..sub.i,j.sup.3 =1 d.sup.ij E.sub.i E.sub.j (E.sub.i =i.sup.th component of radiation source electric field) does not vanish. If the crystal has inversion symmetry, the lowest order non-linear polarization contribution is cubic in the electric field strengths, and this crystal will produce only third or higher harmonics of the initial frequency. Beam or parametric production of harmonics has been demonstrated in crystals such as LiNbO.sub.3, Ba.sub.2 Na(NbO.sub.3).sub.5, LiO.sub.3, KDP, KTiOPO.sub.4 and K.sub.x Rb.sub.1-x TiOPO.sub.4. Laser lines for which SHG has been demonstrated include .lambda.=0.946 .mu.m, 1.064 .mu.m and 1.318 .mu.m (Nd.sup.3+ :YAG) and .lambda.=0.694 .mu.m (ruby).